OBJECTIVE: Acute lower respiratory tract infections are the most common cause of hospital admission in pediatrics. A number of admitted patients need invasive mechanical pulmonary ventilation (IMPV). This study aimed to evaluate prognostic factors for IMPV in infants admitted due to acute lower respiratory infection. METHODS: A prospective cohort study was conducted from April to September, 2004, in two university hospitals of the Campinas metropolitan area, São Paulo, Brazil. One hundred, fifty-two infants were enrolled. Epidemiological and clinical data were recorded at admission and follow-up. Two groups were analyzed, according to the need of IMPV, with a comparison of prognostic factors. Association between risk factors and the outcome were studied and assessed by Relative Risk (RR), with confidence intervals of (95%CI). RESULTS: Twenty-one patients (13.81%) needed IMPV. Factors significantly associated with IMPV on admission were: age < 3 months (RR=2.35, 95%CI:1.06-5.22), breast feeding < 1 month (RR=3.15, 95%CI:1.35-7.35) and cyanosis (RR=7.55, 95%CI:5.01-11.36). In the IMPV group, increased risks for hospitalization > 10 days (RR=13.69, 95%CI:4.92-38.09), oxygen therapy > 10 days (RR=13.57, 95%CI:5.41-34.03), antibiotic usage (RR=3.03, 95%CI:1.34-6.89) and readmission (RR=5.23, 95%CI:2.12-12.91) were observed. CONCLUSION: The associations between need of IMPV and early age, reduced breast feeding and cyanosis demonstrate diminished physiological reserves in the young infant with lower respiratory infection. These patients require prolonged and intensive hospital support and readmission.